An Overview of the Calorimetric Purity Measurement

Abstract

The well-known effect of impurities on the melting behavior of crystalline compounds provides the basis for the determination of the purity of these compounds by differential scanning calorimetry (DSC). In the Perkin-Elmer DSC method for the determination of the purity of crystalline compounds, the van't Hoff equation is used to describe the rate of melting of a compound as a function of sample temperature. The presence of an impurity will broaden the melting peak of the compound and lower the final melting point from To, the melting point of a 100% pure compound, to Tm, the melting point of the impure mixture. If careful sample preparation and slow scanning rates are employed to approach thermodynamic equilibrium, accurate estimation of the true purity of a compound can be determined. The recommended DSC procedure involves the use of a relatively small sample size consistent with homogeneity, the tightest possible encapsulation to minimize temperature gradients within the sample, and the slowest possible scanning rate to approach equilibrium conditions. Also, it is recommended that the melting curve be analyzed over a range of fraction melted (usually 10 to 50%), where the curve is least sensitive to possible gradients in the instrument-sample system.